1. Field of the Invention
The present invention relates to a process for chemical dissolution of a crystalline material. It relates specifically to the chemical machining of piezoelectric materials.
2. Description of the Background
The proliferation of radiocommunication devices, the basic device of which is a piezoelectric resonator, has brought about a rapid development of piezoelectric materials. Quartz (SiO.sub.2) is the most widely used material at this time.
A resonator is made from a section of piezoelectric material. At least one electrode is placed on each main side of the section. The resonator is characterized by its resonance frequency. This frequency corresponds to a specifically determined thickness of the section, between two electrodes. The surface condition of the section should be as favorable as possible, so as not to disrupt the frequency response of the resonator. To give the section the desired thickness, the section is machined at least in its center portion. Various machining processes are known. The higher the frequency of the resonator, the lesser the thickness of the section. At low frequency, machining may be mechanical. The section is ground with an abrasive. For example, for the AT cut of the quartz, a thickness of 40 .mu.m corresponds approximately to a frequency of 40 MHz. For higher frequencies, it no longer is possible to perform mechanical machining. An ionic process may be used. The section is bombarded with accelerated argon ions. The process yields good results from the point of view of the surface of the section, but is costly.
A dissolution process, which uses, in solution, a solvent of the material to be acted upon, also may be employed. For example, a quartz section is immersed in a solution of hydrofluoric acid (HF) or ammonium bifluoride (NH.sub.4 HF.sub.2). This process is less costly than ionic machining, but it is not satisfactory from the point of view of the condition of the surface of the section and thus of electrical behavior of the resonator.
Indeed, the crystalline materials do not have a perfect structure. They inevitably contain structural flaws such as dislocations which outcrop on the surface of the material. Dissolution by the solvent preferably starts at the site of the dislocations. Pits (alternatively "etch pits") then form on the surface of the material. They form grooves and turn into channels (alternatively "etch channels") which completely traverse the section. In short, the surface condition of the section is not improved. The pits and channels bring about serious disruptions in the area of frequency response of the resonator. This process can be used only with high-quality crystalline sections originating from high-purity crystals. A need therefore continues to exist for a method of improving the surface quality of a quartz resonator.